St. Anselm's Proslogion, with A Reply on Behalf of the Fool by Gaunilo and The Author's Reply to Gaunilo

For the philosophically minded

This is the work in which Anselm (a midieval church father) presents his ontological argument for the existence of God. It's one of the most debated philosophical arguments for the existence of God in history. If you're new to phiosophy, you'd better have someone you can ask about after reading this. It's really difficult to grasp. But aside from that, Anselm has a lot of good things to say, and I found that I really liked his writing style. Whether for the sake of philosophy or theology, this is a good book.